Steel wire with excellent corrosion resistance and appearance after processing

ABSTRACT

To provide a steel wire rod having a lubricating coating, which can achieve both corrosion resistance such as long-term rust prevention property, and excellent appearance after heading. 
     The steel wire rod of the present disclosure includes a lubricating coating on a surface thereof, wherein the lubricating coating includes silicon (A), tungsten (B) and an alkali metal salt of fatty acid (C), a dry mass ratio of (B)/(A) is in a range of 1.3 to 18 and a dry mass ratio of (C)/{(A)+(B)} is in a range of 0.14 to 2.0, and the lubricating coating contains no phosphorus.

TECHNICAL FIELD

The present disclosure relates to a steel wire rod having a lubricating coating containing no phosphorus on a surface.

BACKGROUND ART

In plastic working of a steel wire and a steel wire rod, friction generated when surfaces of metals (particularly a die and a workpiece) are violently rubbed against each other may cause an increase in working energy, heat generation, and seizure phenomenon. Thus, there have been used various lubricants which aim to reduce a friction force. Oils and soaps have been used as the lubricant for a long time, and the friction force has been reduced by supplying them to a friction surface to form a fluid lubricating coating. However, in plastic working in which sliding occurs under high surface pressure involving significant heat generation due to an increase in surface area, a seizure phenomenon is likely to generate due to shortage of lubrication and lubricating coating disruption. Therefore, there has been popularized technology in which a surface of a metal material is coated in advance with a solid coating, for example, inorganic coatings such as a borate coating and a phosphate crystal coating, which exist at an interface between a die and a workpiece and is therefore less likely to cause lubricating coating disruption even under high surface pressure, thus making it possible to avoid direct contact between metals. Such lubricating coating has sufficient coating strength. In particular, a composite coating composed of a zinc phosphate coating and a soap layer (hereinafter sometimes referred to as a chemical conversion coating) has widely been employed because of having high workability and corrosion resistance.

In recent years, there have been surging a wide range of requirements for a solid coating, for example, further reduction in working energy and increase in working degree, coping with a hard-to-work material, environmental protection of a coating process (for example, a phosphatizing treatment has an environmental conservation problem because of generation of numerous industrial wastes such as sludge), and taking measures to phosphorizing of a bolt (if phosphorus in a coating component remains during a heat treatment after heading of a high strength bolt, phosphorus enters into a steel, thus causing brittle fracture). While global environment conservation is taken into consideration to these requirements, a solid coating having high lubricity has been developing. This technology enables formation of a coating having high lubricity by a simple step of only applying an aqueous plastic working lubricant to a surface of a workpiece, followed by drying.

Patent Document 1 discloses an aqueous lubricating coating agent for plastic working of a metal material, which is a composition comprising a water-soluble inorganic salt (A) and a wax (B) dissolved or dispersed in water, wherein a solid component weight ratio (B)/(A) is in a range of 0.3 to 1.5; and a coating forming method thereof.

Patent Document 2 discloses an aqueous lubricating coating agent for plastic working of a metal material, comprising an alkali metal borate (A), wherein the alkali metal borate (A) contains lithium borate, a molar ratio of lithium to the entire alkali metal in the alkali metal borate (A) is in a range of 0.1 to 1.0, and also a molar ratio (B/M) of boric acid B to an alkali metal M of the alkali metal borate (A) is in a range of 1.5 to 4.0; and a coating forming method thereof. It is considered that this technology suppresses crystallization of a coating caused by moisture absorption of the coating, thus enabling formation of a coating having not only workability but also high corrosion resistance.

Patent Document 3 discloses a water-soluble lubricant for non-phosphorus based plastic working, comprising an inorganic solid lubricant as a component A, a wax as a component B, and a water-soluble inorganic metal salt as a component C, wherein a solid component dry mass ratio of the component A to the component B (component A/component B) is in a range of 0.1 to 5, and a solid component dry mass ratio of the component C to the total amount of the component A, the component B, and the component C (component C/(component A+component B+component C)) is in a range of 1 to 30%. It is considered that this technology is directed to a lubricant containing no phosphorus and enables realization of corrosion resistance equal to that of a chemical conversion coating.

Patent Document 4 discloses an aqueous lubricating coating agent comprising a water-soluble inorganic salt (A), at least one lubricant (B) selected from molybdenum disulfide and graphite, and a wax (C), these components being dissolved or dispersed in water, wherein (B)/(A) is in a range of 1.0 to 5.0 in terms of a solid component weight ratio, and (C)/(A) is in a range of 0.1 to 1.0 in terms of a solid component weight ratio; and a coating forming method thereof. It is considered that this technology enables realization of high workability having the same level as that of a chemical conversion coating by mixing a conventional aqueous lubricating coating agent with molybdenum disulfide and/or graphite.

Patent Document 5 discloses a coating forming agent comprising a silicate (A), a polycarboxylate (B), a hydrophilic polymer and/or a hydrophilic organic lamellar structure (C), and a molybdate and/or a tungstate (D), a dry mass ratio of each component being a predetermined ratio.

As mentioned in Patent Documents 1 to 5, the water-soluble inorganic salt is an essential component in the solid coating of the aqueous lubricating coating agent. The reason is that the lubricating coating composed of the water-soluble inorganic salt has sufficient coating strength and, as mentioned above, the lubricating coating exists at an interface between a die and a workpiece and is therefore less likely to cause lubricating coating disruption even under high surface pressure, thus making it possible to avoid direct contact between metals. Therefore, in the aqueous lubricating coating agent, it is possible to maintain a satisfactory lubricated state during plastic working by using a solid coating composed of a water-soluble inorganic salt and a water-soluble resin in combination with an appropriate lubricant capable of reducing a friction coefficient.

A description will be made of coating formation mechanism of the aqueous lubricating coating composed of a water-soluble component. A water-soluble inorganic salt of a water-soluble component is in a state of being dissolved in water in a lubricating treatment solution and, when a lubricant is applied on a surface of a metal material and then dried, water as a solvent is vaporized to form a lubricating coating. In that case, the water-soluble inorganic salt is precipitated as a solid substance on the surface of the metal material to form a solid coating. The solid coating thus formed has a coating strength capable of enduring plastic working, and exhibits satisfactory lubricity during plastic working by mixing with an appropriate lubricant capable of reducing a friction coefficient.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: WO 02/012420 A

Patent Document 2: JP 2011-246684 A

Patent Document 3: JP 2013-209625 A

Patent Document 4: WO 02/012419 A

Patent Document 5: JP 2002-363593 A

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the lubricating coatings of Patent Documents 1 to 5, rust prevention property over a long term of two or more months is drastically inferior as compared with the above-mentioned chemical conversion coatings, thus failing to enhance to a practical level. This is because a main component of the coating is a water-soluble component and therefore easily absorbs or transmits moisture in the atmosphere, leading to easy contact between a steel rod and moisture. In Patent Document 2, corrosion resistance is improved by suppressing crystallization of the coating due to moisture absorption, however, moisture absorption itself is not suppressed, thus failing to obtain sufficient corrosion resistance. It was mentioned that the aqueous lubricating coating mentioned in Patent Document 3 exhibited corrosion resistance, which is equal to or better than that of the chemical conversion coating, in a corrosion resistance test in a laboratory in which rusting is accelerated using a thermo-hygrostat. Commonly, the lubricating coating is actually used in the environment where dusts and powders, and mists of a pickling agent are adhesible. In such severe environment, corrosion resistance is actually inferior as compared with the chemical conversion coating. As mentioned above, there has never been an aqueous lubricating coating containing no phosphorus, having rust prevention property which is equal to or better than that of the chemical conversion coating.

Examples of the water-soluble inorganic salt capable of obtaining comparatively high corrosion resistance include an alkali metal salt of a silicate (hereinafter sometimes referred to as a silicate) and an alkali metal salt and/or an ammonium salt of a tungstate (hereinafter sometimes referred to as a tungstate). These water-soluble inorganic salts are mentioned in Patent Document 1, Patent Document 4, and Patent Document 5. However, they are far inferior in practical corrosion resistance as compared with the chemical conversion coating.

The water-soluble silicate has a property that is less likely to transmit moisture among the water-soluble inorganic salts and also has very high adhesion to a base material. Because of this property, it is a material that can exhibit comparatively high corrosion resistance, but not as much as the chemical conversion coating. This is because the water-soluble silicate is crosslinked to form a network structure in a coating formation process in which water as a solvent of a lubricant is vaporized. However, because of this network structure, the coating of the water-soluble silicate is too brittle as a lubricating coating. Therefore, when the base material is worked, it is sometimes impossible to sufficiently conform because of cracks of the coating, leading to decrease in coating retention amount.

The water-soluble tungstate is less likely to absorb moisture from external air when a coating is formed. This is because granular crystals are formed when the water-soluble tungstate forms a coating. Further, the water-soluble tungstate has a property that forms a passive coating having a self-repair function on a surface of a steel rod, and use of the water-soluble tungstate as the coating component enables expectation of formation of a coating having high corrosion resistance. However, the water-soluble tungstate is crystalline and is therefore inferior in adhesion to a material and cannot form a uniform coating, thus failing to obtain corrosion resistance and workability as expected. For example, it is possible to enhance adhesion and uniformity of a coating by adding a synthetic resin component in a lubricant, but the corrosion resistance is drastically inferior as compared with a chemical conversion coating.

It is possible for the aqueous lubricating coating agent mentioned in Patent Document 4 to obtain workability, which is equal to or better than that of a chemical conversion coating, even during severe working by inclusion of molybdenum disulfide and/or graphite. However, the resulting coating is inferior in corrosion resistance as compared with the lubricating coatings of Patent Documents 1 to 3.

In Patent Document 5, a coating treatment agent containing a silicate (A) as a main component and containing excessively large amount of an anti-corrosive agent (D) is inferior in lubricity since seizure occurs under high extrusion load. Therefore, it becomes difficult to perform stable operation, thus failing to obtain sufficient long-term rust prevention property.

Furthermore, when steel rods having a lubricating coating mentioned in Patent Documents 1 to 5 is subjected to heading, pushing-in of a coating to a steel rod occurs according to the method of heading and the shape after heading, leading to poor appearance of the product. Such lubricating coating is likely to cause coarse distribution of a lubricant component in the coating, leading to a local difference in a friction state between the place where a lubricant content is high and the place where a lubricant content is low during heading. As a result, the deformation amount locally differs when a steel rod is worked, so that a phenomenon similar to a so-called stick-slip phenomenon occurs. Particularly, pushing-in of the coating occurs at the place where friction is locally high, causing poor appearance such as formation of scaly patterns. An inorganic salt to be mixed in a lubricating coating is comparatively hard, so that push-in traces are likely to be generated by a surface pressure during heading in a steel rod. These patterns and push-in traces can be visually confirmed with ease, thus drastically impair the appearance of the product.

As mentioned above, it was impossible for the aqueous lubricating coating which can achieve both high corrosion resistance over a long term of about two or more months comparable to that of the chemical conversion coating even in a practical environment, and excellent appearance even after heading.

Thus, it is an object of the embodiment of the present invention to provide a steel wire rod having a lubricating coating which can achieve both corrosion resistance such as long-term rust prevention property, and excellent appearance after heading.

Means for Solving the Problems

The inventors of the present invention have intensively studied on a steel wire rod having a lubricating coating containing no phosphorus on a surface so as to solve the above problems, and found that it is possible to simultaneously obtain high corrosion resistance which has never been achieved by each component alone, and excellent appearance after heading which has never been obtained by mixing lubricants such as wax, by mixing a predetermined ratio of an alkali metal salt of fatty acid among various compounds used as a lubricant in a lubricating coating in which a ratio of silicon derived from a water-soluble silicate to tungsten derived from a water-soluble tungstate, i.e. a dry mass ratio of tungsten/silicon is controlled to a predetermined ratio, and thus the present invention has been completed.

The present invention was structured in the following manner so as to solve the above problems.

The steel wire rod of the embodiment of the present invention includes a lubricating coating on a surface thereof, the lubricating coating including silicon (A), tungsten (B) and an alkali metal salt of fatty acid (C), a dry mass ratio of (B)/(A) being in a range of 1.3 to 18, a dry mass ratio of (C)/{(A)+(B)} being in a range of 0.14 to 2.0, and the lubricating coating containing no phosphorus.

It is preferred that the silicon is derived from a water-soluble silicate, and the tungsten is derived from a water-soluble tungstate.

It is preferred that the silicon is derived from at least one selected from the group consisting of lithium silicate, sodium silicate, and potassium silicate, and the tungsten is derived from at least one selected from the group consisting of lithium tungstate, sodium tungstate, potassium tungstate, and ammonium tungstate.

It is preferred that the lubricating coating further includes a lubricant (D) other than the alkali metal salt of fatty acid (C), and a dry mass ratio of {(C)+(D)}/{(A)+(B)} is in a range of 0.14 to 2.0.

It is preferred that the lubricant (D) is at least one selected from the group consisting of wax, polytetrafluoroethylene, fatty acid metal soap, fatty acid amide, molybdenum disulfide, tungsten disulfide, graphite, and melamine cyanurate.

It is preferred that the lubricating coating further includes a resin (E), and a dry mass ratio of (E)/{(A)+(B)} exceeds 0 and 1.4 or less.

It is preferred that the resin (E) is at least one selected from the group consisting of a vinyl resin, an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a cellulose derivative, a polymaleic acid, and a polyester resin.

It is preferred that a mass of a coating per unit area of the lubricating coating is in a range of 1.0 to 20 g/m².

Effects of the Invention

In the steel wire rod of the present disclosure, a lubricating coating is structured in the manner mentioned above, thus obtaining a steel wire rod which has excellent corrosion resistance such as long-term rust prevention property, and satisfactory appearance after heading. The lubricating coating of the present disclosure is far better than a conventional aqueous lubricating coating in that all of these performances are equal to or better than those of steel wire rods having a chemical conversion coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the procedure of heading in Examples.

MODE FOR CARRYING OUT THE INVENTION

The steel wire rod of the embodiment of the present invention is characterized by including a lubricating coating on a surface thereof, the lubricating coating including silicon (A), tungsten (B) and an alkali metal salt of fatty acid (C), a dry mass ratio of (B)/(A) being in a range of 1.3 to 18, a dry mass ratio of (C)/{(A)+(B)} being in a range of 0.14 to 2.0, and the lubricating coating containing no phosphorus.

Steel used in the steel wire rod of the embodiment of the present invention include a carbon steel, an alloy steel, and a special steel. Examples of such steel include a mild steel having a carbon content of 0.2% by mass or less (not including 0% by mass), a carbon steel having a carbon content of exceeding 0.2% by mass and 1.5% by mass or less. The steel used in the steel wire rod also includes an alloy or special steel in which at least one selected from silicon, manganese, phosphorus, sulfur, nickel, chromium, copper, aluminum, molybdenum, vanadium, cobalt, titanium, and zirconium is added to the mild steel or carbon steel according to the application.

In the present disclosure, the steel wire rod commonly refers to those obtained by forming a steel into a wire rod through hot working. A steel wire is also included in the steel wire rod of the present disclosure. The steel wire is obtained by further subjecting the steel wire rod to a working treatment. Examples of the working treatment include a wire drawing treatment, a heading treatment, and a forging treatment. Specific examples thereof include those obtained by drawing a steel wire rod into a wire having a specified size (wire diameter, circularity, etc.), those obtained by heading after wire drawing, and those obtained by forging the steel wire rod. The steel wire also includes those obtained by further subjecting the product after the working treatment to surface treatments such as heat treatment and/or a plating treatment. Specific examples thereof include those obtained by machining into a product after the heading or forging, and subjecting the product to a heat treatment, or those obtained by further subjecting to a plating treatment after the heat treatment.

The steel wire rod of the embodiment of the present invention is not particularly limited as long as it is excellent in corrosion resistance and appearance after heading because of having the below-mentioned lubricating coating, and a coating film, namely, an undercoating may be further formed between a surface of the steel wire rod and the lubricating coating. Both of these coatings may be a single layer, or a layer composed of two or more layers.

Both the lubricating coating and the undercoating used in the embodiment of the present invention contain no phosphorus, and a lubricating coating agent used for formation of the lubricating coating does not contain a component containing phosphorus. However, in the embodiment of the present invention, it is not excluded that a component containing phosphorus is inevitably included in a coating of a surface of the steel wire rod in the operation process. Namely, although phosphorus may cause contamination as inevitable impurity in the actual operation, there is little possibility that phosphorus causes brittle fracture of a steel wire rod when the content of phosphorus is about 1% by mass or less, and thus it is possible to consider that phosphorizing does not occur.

A description will be made in order below from each component and composition of the lubricating coating in the steel wire rod of the embodiment of the present invention.

The steel wire rod of the embodiment of the present invention includes a lubricating coating on a surface thereof, the lubricating coating including silicon (A), tungsten (B) and an alkali metal salt of fatty acid (C), a dry mass ratio of (B)/(A) being in a range of 1.3 to 18, and a dry mass ratio of (C)/{(A)+(B)} being in a range of 0.14 to 2.0. Formation of a lubricating coating including the above components in a ratio controlled in the above range on a surface of a steel wire rod enables simultaneous achievement of high corrosion resistance and excellent appearance after heading.

For example, the below-mentioned water-soluble silicate and water-soluble tungstate are composited to form a lubricating coating, the water-soluble tungstate is incorporated into a network structure formed of the water-soluble silicate. As mentioned above, the water-soluble tungstate has drawbacks that it forms a crystalline coating. However, it becomes possible for the water-soluble tungstate to exist uniformly and finely by incorporating into the network structure of the water-soluble silicate. Whereby, a passive film having a property of being less likely to transmit moisture of the water-soluble silicate and a self-repair function of the water-soluble tungstate is formed, leading to a remarkable improvement in corrosion resistance.

Examples of the influence of the water-soluble tungstate on the water-soluble silicate include an increase in coating retention amount of the working portion involved in an improvement in coating conformability. As mentioned above, the water-soluble silicate is inferior in coating conformability since a firm continuous film is formed by polymerization of the water-soluble silicate. The composited water-soluble tungstate interposes in the network structure of the water-soluble silicate, whereby, formation of a firm network structure is appropriately suppressed, and coating conformability is improved, thus enabling an increase in coating retention amount.

Particularly, to ensure satisfactory corrosion resistance, a dry mass ratio of tungsten (B)/silicon (A) is 1.3 or more, preferably 1.8 or more, and more preferably 2.0 or more. The dry mass ratio is 18 or less, preferably 10 or less, and more preferably 5.4 or less. If the dry mass ratio of B/A is less than 1.3, sufficient corrosion resistance cannot be obtained and the coating retention amount of the working portion decreases. This is because the amount of the tungstate relatively decreases, thus failing to sufficiently form a passive film, while the amount of the silicate relatively increases to form a firm network structure. If a dry mass ratio of tungsten/silicon is more than 18, the thus formed coating cannot achieve sufficient corrosion resistance. This is because the amount of the silicate relatively decreases, thus making it easier to transmit moisture, while crystals of tungsten are precipitated, thus degrading adhesion and uniformity of the coating. In the embodiment of the present invention, a dry mass ratio of tungsten/silicon is based on a ratio of tungsten element derived from a water-soluble tungstate to silicon element derived from a water-soluble silicate in the coating, and can be calculated as mentioned later.

In the embodiment of the present invention, it is suitable that silicon (A) is derived from a water-soluble silicate and tungsten (B) is derived from a water-soluble tungstate. A dry mass ratio of tungsten (B)/silicon (A) in the below-mentioned Examples is based on a ratio of tungsten element derived from the water-soluble tungstate in the lubricating coating to a silicon element derived from the water-soluble silicate in the lubricating coating and can be calculated, for example, using inductively coupled plasma or fluorescent X-ray spectroscopy.

When the silicon (A) is derived from a water-soluble silicate (a) and the tungsten (B) is derived from a water-soluble tungstate (b), a dry mass ratio (b)/(a) thereof is 0.7 or more, preferably 0.9 or more, and more preferably 1.1 or more. The dry mass ratio is 10 or less, preferably 6.0, and more preferably 3.0 or less.

Examples of the water-soluble silicate include lithium silicate, sodium silicate, and potassium silicate. These water-soluble silicates may be used alone, or two or more water-soluble silicates may be used in combination.

Examples of the water-soluble tungstate include lithium tungstate, sodium tungstate, potassium tungstate, and ammonium tungstate. These water-soluble tungstates may be used alone, or two or more water-soluble tungstates may be used in combination.

Next, description will made of an alkali metal salt of fatty acid (C). The alkali metal salt of fatty acid (C) is mixed to reduce friction of a lubricating coating and to prevent poor appearance after heading. The alkali metal salt of fatty acid (C) has slight water solubility and, in the process for forming a lubricating coating, the dissolved alkali metal salt of fatty acid (C) is precipitated finely and uniformly. As mentioned above, a conventional lubricating coating is likely to cause coarse distribution of a lubricant component in the coating, leading to a local difference in a friction state between the place where a lubricant content is high and the place where a lubricant content is low during heading. Unlike other lubricant components, the alkali metal salt of fatty acid (C) can exist finely and uniformly in the coating, so that a local difference in a friction state is less likely to occur, thus obtaining satisfactory appearance (heading appearance) after heading. Mixing of the alkali metal salt of fatty acid (C) exerts the effect of softening a lubricating coating, and thus the lubricating coating becomes harder to push in the steel wire rod.

To effectively exert the addition effect of the above-mentioned alkali metal salt of fatty acid (C), (C)/{(A)+(B)}, which is a dry mass ratio of silicon (A), tungsten (B), and the alkali metal salt of fatty acid (C), is 0.14 or more, preferably 0.2 or more, and more preferably 0.4 or more. The dry mass ratio is 2.0 or less, and preferably 1.5 or less. If the dry mass ratio is less than 0.14, it becomes impossible to obtain satisfactory heading appearance. Meanwhile, if the dry mass ratio is more than 2.0, each amount of silicon (A) and tungsten (B) relatively decreases, thus causing degradation of corrosion resistance, degradation of the seizure resistance of the coating during heading, and reduction in service life of the die caused thereby.

When the silicon (A) is derived from a water-soluble silicate (a) and the tungsten (B) is derived from a water-soluble tungstate (b), a dry mass ratio (C)/{(a)+(b)} is 0.043 or more, preferably 0.062 or more, and more preferably 0.09 or more. The dry mass ratio is 0.95 or less, and more preferably 0.8 or less.

The alkali metal salt of fatty acid (C) used in the embodiment of the present invention means an alkali metal salt (for example, sodium salt, potassium salt, lithium salt) of a long-chain fatty acid having 12 or more carbon atoms (higher fatty acid). A hydrocarbon group constituting fatty acid may be either linear or branched. Examples of the alkali metal salt of fatty acid (C) include sodium myristate, potassium myristate, lithium myristate, sodium palmitate, potassium palmitate, lithium palmitate, sodium stearate, potassium stearate, lithium stearate, sodium 12-hydroxystearate, potassium 12-hydroxystearate, and lithium 12-hydroxystearate. These alkali metal salts of fatty acid may be used alone, or two or more alkali metal salts of fatty acid may be used in combination.

Description was made of basic components of a lubricating coating constituting the steel wire rod of the embodiment of the present invention.

The steel wire rod of the embodiment of the present invention may further include a lubricant (D) and/or a resin (E) other than the fatty acid alkali metal salt (C) in the lubricating coating.

Among these, the lubricant (D) itself has slipperiness, and has a function of reducing a friction force. In general, if the friction force increases during plastic working, an increase in working energy, heat generation, seizure, and the like occur. If the lubricant (D) is included in a lubricating coating of the steel wire rod of the embodiment of the present invention, the lubricant exists in the lubricating coating in the form of a solid, thus suppressing an increase in friction force. Examples of the lubricant (D) having such function and property include wax, polytetrafluoroethylene, fatty acid metal soap, fatty acid amide, molybdenum disulfide, tungsten disulfide, graphite, and melamine cyanurate. These lubricants may be used alone, or two or more lubricants may be used in combination.

Specific examples of the wax include polyethylene wax, paraffin wax, microcrystalline wax, polypropylene wax, and carnauba wax. Specific examples of the fatty acid metal soap include calcium stearate, zinc stearate, barium stearate, and magnesium stearate. The fatty acid amide is an amide compound having two fatty acids, and specific examples thereof include ethylenebislauric acid amide, ethylenebisstearic acid amide, ethylenebisbehenic acid amide, N,N′-distearyladipic acid amide, ethylenebisoleic acid amide, ethylenebiserucic acid amide, hexamethylenebisoleic acid amide, and N,N′-dioleyladipic acid amide.

When the lubricating coating in the embodiment of the present invention further includes the lubricant (D), to effectively exert the addition effect of the lubricant (D), a dry mass ratio of {(C)+(D)}/{(A)+(B)} is preferably 0.14 or more, more preferably 0.2 or more, and still more preferably 0.4 or more. The dry mass ratio is preferably 2.0 or less, and more preferably 1.5 or less. If the dry mass ratio is less than 0.14, the content of the lubricant (D) is too small to exhibit the above-mentioned performances. Meanwhile, if the dry mass ratio is more than 2.0, each amount of silicon and tungsten relatively decreases, leading to degradation of corrosion resistance.

The resin (E) enables the binder effect, an improvement in adhesion between a base material and a coating, imparting of leveling property by the thickening effect, and stabilization of a dispersion component. Examples of the resin having such function and property include a vinyl resin, an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a cellulose derivative, a polymaleic acid, and a polyester resin. A methacrylic resin is also included in the acrylic resin. These resins may be either polymers or copolymers. Examples of the copolymer include a copolymer of vinyl-acrylic acid, a copolymer of vinyl-epoxy, a copolymer of vinyl-urethane, a copolymer of vinyl-phenol, a copolymer of vinyl-(anhydrous) maleic acid, a copolymer of acrylic acid-epoxy, a copolymer of acrylic acid-urethane, a copolymer of acrylic acid-phenol, a copolymer of acrylic acid-(anhydrous) maleic acid, a copolymer of epoxy-urethane, a copolymer of epoxy-phenol, a copolymer of epoxy-(anhydrous) maleic acid, a copolymer of urethane-phenol, a copolymer of urethane-(anhydrous) maleic acid, a copolymer of phenol-(anhydrous) maleic acid, a copolymer of olefin-acrylic acid, a copolymer of olefin-epoxy, a copolymer of olefin-urethane, a copolymer of olefin-phenol, and a copolymer of olefin-(anhydrous) maleic acid. These copolymers may be used alone, or two or more copolymers may be used in combination.

When the resin (E) is further included in the lubricating coating, to effectively exert the addition effect of the resin (E), a dry mass ratio of (E)/{(A)+(B)} is preferably 0.01 or more, and more preferably 0.05 or more. The dry mass ratio is preferably 1.4 or less, and more preferably 0.9 or less. If the dry mass ratio is less than 0.01, the above-mentioned action may not be sufficiently exerted. Meanwhile, if the dry mass ratio is more than 1.4, poor appearance is likely to occur after heading.

Furthermore, the lubricating coating constituting the embodiment of the present invention can be further mixed with, in addition to the above-mentioned basic components (silicon, tungsten, an alkali metal salt of fatty acid) and selective components (a resin, a lubricant other than an alkali metal salt of fatty acid), a viscosity modifier. Whereby, when a lubricating coating agent is coated on a steel wire rod which is subjected to a working treatment (which means a steel wire rod requiring a working treatment, namely, a steel wire rod before a working treatment), leveling property and thixotrophy are imparted, thus making it possible to ensure a uniform coated state. Specific examples of such viscosity modifier include smectite-based clay minerals such as montmorillonite, sauconite, beidellite, hectorite, nontronite, saponite, iron-rich saponite, and stevensite; and inorganic thickeners such as pulverized silica, bentonite, and kaolin.

The lubricating coating may further include water-soluble salts to improve adhesion. Type of the water-soluble salt is not particularly limited, and either or both of an inorganic salt and an organic salt can be used. Examples of the inorganic salt include sulfates such as sodium sulfate and potassium sulfate; and borates such as sodium metaborate, potassium metaborate, and ammonium metaborate. Examples of the organic salt include salts of formic acid, acetic acid, butyric acid, oxalic acid, succinic acid, lactic acid, ascorbic acid, tartaric acid, citric acid, malic acid, malonic acid, maleic acid, phthalic acid, and the like, with alkali metals, alkali earth metals, and the like.

The lubricating coating of the steel wire rod of the embodiment of the present invention can be imparted with high corrosion resistance before and after wire drawing, and may be mixed with other water-soluble rust preventives and inhibitors for the purpose of further improving corrosion resistance. It is possible to use known rust preventives and inhibitors, and examples thereof include various organic acids such as oleic acid, dimer acid, tartaric acid, and citric acid; various chelating agents such as EDTA, NTA, HEDTA, and DTPA; mixed components of alkanolamine such as triethanolamine; amine salts of p-t-butylbenzoic acid; a carboxylic acid amine salt, a dibasic acid amine salt, an alkenylsuccinic acid, and a water-soluble salt thereof; and aminotetrazole and a water-soluble salt thereof. These rust preventives and inhibitors may be used alone, or two or more rust preventives and inhibitors may be used in combination.

The lubricating coating constituting the steel wire rod of the embodiment of the present invention can be fabricated by adding the above-mentioned components (water-soluble silicate which is a typical source of supply of silicon, water-soluble tungstate which is typical source of supply of tungsten, and essential components of an alkali metal salt of fatty acid; and, if necessary, lubricants other than an alkali metal salt of fatty acid, resins, viscosity modifiers, and water-soluble salts) to a liquid medium, followed by mixing to prepare a lubricating coating agent, and coating the lubricating coating agent on a surface of a steel wire rod which is subjected to a working treatment.

The amount of the water-soluble silicate is preferably exceeding 5% by mass, more preferably 10% by mass or more, and still more preferably 15% by mass or more, and is also preferably 58% by mass or less, more preferably 52% by mass or less, and still more preferably 45% by mass or less, based on 100% by dry mass of the total amount of a component a, a component b, a component C, a component D, and a component E in the lubricating coating agent.

The amount of the water-soluble tungstate is preferably 10% by mass or more, more preferably 15% by mass or more, and still more preferably 20% by mass or more, and is also preferably less than 88% by mass, more preferably 85% by mass or less, and still more preferably 80% by mass or less, based on 100% by dry mass of the total amount of a component a, a component b, a component C, a component D, and a component E in the lubricating coating agent.

The amount of the alkali metal salt of fatty acid is preferably exceeding 3% by mass, more preferably exceeding 7% by mass, still more preferably 12% by mass or more, and particularly preferably 16% by mass or more, and is also preferably 50% by mass or less, more preferably 40% by mass or less, and still more preferably 30% by mass or less, based on 100% by dry mass of the total amount of a component a, a component b, a component C, a component D, and a component E in the lubricating coating agent.

Here, if the amount of the water-soluble silicate is 5% by mass or less and the amount of the water-soluble tungstate is 88% by mass or more, sufficient long-term rust prevention property cannot be obtained, and poor appearance after heading occurs. This is because the amount of the water-soluble silicate relatively decreases, thus making it easier to transmit moisture, while crystals of tungsten are precipitated, thus degrading adhesion and uniformity of the coating. Meanwhile, if the amount of the water-soluble silicate exceeds 58% by mass and the amount of the water-soluble tungstate is less than 10% by mass, neither sufficient corrosion resistance nor satisfactory appearance after heading can be obtained. This is because the amount of the tungstate relatively decreases, thus failing to sufficiently form a passive film, while the amount of the water soluble silicate relatively increases to form a firm network structure.

If the amount of the alkali metal salt of fatty acid exceeds 50% by mass, each amount of silicon (A) and tungsten (B) relatively decreases, thus degrading corrosion resistance. Meanwhile, if the amount of the alkali metal salt of fatty acid is 3% by mass or less, poor appearance is likely to occur after heading.

To further improve lubricity, seizure resistance, and corrosion resistance, a dry lubricant may be adhered onto the lubricating coating. There is no limitation on type of the dry lubricant and it is possible to use, for example, a common lubricating powder and/or wire drawing powder which contains, as main components, higher fatty acid soap, borax, lime, and molybdenum disulfide.

In the embodiment of the present invention, a liquid medium (solvent, dispersion medium) in a lubricating coating agent for forming a lubricating coating is water. To shorten the drying time of the lubricating coating agent in the drying step, it is possible to mix an alcohol having a boiling point lower than that of water.

To enhance stability of the solution, the lubricating coating agent may contain a water-soluble strong alkali component. Specific examples thereof include lithium hydroxide, sodium hydroxide, and potassium hydroxide. These water-soluble strong alkali components may be used alone, or two or more water-soluble strong alkali components may be used in combination.

A method for producing a steel wire rod according to the embodiment of the present invention will be described below. The production method includes a cleaning step of a steel wire rod which is subjected to a working treatment, a step of producing (treating) a lubricating coating, a drying step, and a working step. After the working step, a step of a heat treatment and/or a surface treatment, and the like may be further performed. Each step will be described below.

Cleaning Step (Pretreatment Step)

Before formation of a coating on a steel wire rod, at least one cleaning treatment selected from the group consisting of shot blasting, sand blasting, wet blasting, peeling, alkali degreasing, and pickling is preferably performed. Cleaning as used herein is performed for the purpose of removing oxide scales grown by annealing, and various contaminations (oils, etc.).

Production Step of Lubricating Coating

In the embodiment of the present invention, there is no particular limitation on a production step of a lubricating coating on a steel wire rod which is subjected to a working treatment, and it is possible to use coating methods such as an immersion method, a flow coating method, and a spraying method. The degree of coating is not particularly limited as long as a surface of the steel wire rod is sufficiently coated with a lubricating coating agent used in the embodiment of the present invention, and also the coating time is not particularly limited. To enhance drying property during coating, the steel wire rod may be brought into contact with the lubricating coating agent after heating to a temperature in a range of 60 to 80° C. The steel wire rod may also be brought into contact with the lubricating coating agent heated to a temperature in a range of 40 to 70° C. By these methods, the drying property may be sometimes improved significantly, thus enabling drying at a normal temperature and reduction in thermal energy loss.

Drying Step

There is a need to dry the lubricating coating agent. Drying may be performed by being left to stand at a normal temperature, or may performed at 60 to 150° C. for 1 to 30 minutes.

Working Step

The steel wire rod having a lubricating coating obtained by performing the production step of a lubricating coating and the drying step falls within the scope of steel wire rod according to the embodiment of the present invention. Even those obtained by subjecting the steel wire rod having a lubricating coating to a working treatment by the working step fall within the scope of steel wire rod according to the embodiment of the present invention as long as they have a lubricating coating. As mentioned above, examples of the working treatment include a wire drawing treatment, a heading treatment, a forging treatment, and the like.

The mass of the coating per unit area of the lubricating coating formed on the steel wire rod is appropriately controlled by the degree of subsequent working. The mass of the coating per unit area is preferably 1.0 g/m² or more, and more preferably 2.0 g/m² or more, and is also preferably 20 g/m² or less, and more preferably 15 g/m² or less. The mass of the coating per unit area can be calculated from a difference in mass between steel wire rods before and after a treatment, and a surface area. To control so as to adjust in a range of the above-mentioned mass of the coating, the solid component mass (concentration) of the lubricating coating agent is appropriately adjusted. In practice, after diluting a high concentration lubricating coating agent with water, the thus obtained dilution is often used. There is no particular limitation on water used for dilution and adjustment, and, for example, pure water, deionized water, tap water, ground water, industrial water, and the like can be used.

Step after Working Step

After the working step, the steel wire rod of the embodiment of the present invention can be further subjected to steps of a heat treatment, a surface treatment such as a plating treatment and the like. The heat treatment is performed for the purpose of imparting strength and/or toughness by making the steel wire rod obtained by working step hard. There is no particular limitation on the method of the heat treatment, and a common method can be employed and includes, for example, common heat treatments such as hardening and tempering; and surface heat treatments such as carburizing and quenching, and nitridization. The plating treatment is performed for the purpose of imparting corrosion resistance, and is mainly applied to those after the heat treatment. There is no particular limitation on the method of the plating treatment, and a common method can be employed and includes, for example, electroplating, hot dipping and the like. There is no particular limitation on type of plating, and common plating can be performed and includes, for example, zinc plating, chromium plating, and nickel plating.

Film Removal Method

In the embodiment of the present invention, film removal can be performed by immersing the lubricating coating formed of the lubricating coating agent in an aqueous alkali cleaner, or spraying the aqueous alkali cleaner. The alkali cleaner is a solution prepared by dissolving a common alkali component such as sodium hydroxide or potassium hydroxide in water and, when the alkali cleaner is brought into contact with the lubricating coating, the lubricating coating dissolves in the cleaning solution, thus making it possible to easily perform film removal. It is also possible to obtain a coating capable of easily falling off by a heat treatment after working. Therefore, alkali cleaning and/or heat treatment enable prevention of contamination and poor plating in the subsequent step caused by insufficient film removal.

Examples

The present disclosure will be more specifically described below by way of Examples and Comparative Examples, together with effects thereof, with respect to a steel wire rod. The present disclosure is not limited to these Examples.

(1-1) Preparation of Aqueous Lubricating Coating Agent

The respective components shown below were mixed with water such that the solid component mass ratio would be those shown in Table 1 to prepare aqueous lubricating coating agents in which steel wire rods or cold rolled steel sheets (SPCC-SD) of Example 1 to 19 and Comparative Examples 1 to 7 are immersed. The amount of water mixed with each component was appropriately adjusted such that the mass of the lubricating coating to be formed would be those shown in Table 1. These lubricating coating agents were mixed with 0.5% by mass of lithium hydroxide. Commercially available reactive soap lubricant was used as a lubricating coating agent in which steel wire rods or cold rolled steel sheets (SPCC-SD) of Comparative Example 8 (Conventional Example) are immersed.

<Water-Soluble Silicate>

(a-1) No. 3 sodium silicate (Na₂O-nSiO₂, n=3) (a-2) Lithium silicate (Li₂O-nSiO₂, n=3.5)

<Water-Soluble Tungstate>

(b-1) Sodium tungstate (b-2) Potassium tungstate

<Alkali Metal Salt of Fatty Acid>

(C-1) Lithium 12-hydroxystearate (C-2) Sodium isohexadecanoate (branched palmitate)

<Lubricant>

(D-1) Anionic polyethylene wax (average particle size of 5 μm) (D-2) Calcium stearate (average particle size of 8 μm)

<Resin>

(E-1) Sodium neutralizing salt of isobutylene-maleic anhydride copolymer (weight average molecular weight of 160,000 to 170,000)

TABLE 1 Composition of aqueous lubricafng coating agent Mass of coating and each ratio of coating component Water- Water- of lubricating coating formed on steel wire rod soluble soluble Alkali metal Mass of silicate tungstate salt of fatty coating (compo- (compo- acid Lubricant Resin per (C)/ (E)/ nent a) nent b) (component C) (component D) (component E) unit area {(A) + {(C) + (D)}/ {(A) + (a-1) (a-2) (b-1) (b-2) (C-1) (C-2) (D-1) (D-2) (E-1) (g/m²) (B)/(A) (B)} {(A) + (B)} (B)} Example 1 25 0 45 0 30 0 0 0 0 10.0 3.24 0.81 0.81 — Example 2 33 0 37 0 18 0 10 0 2 7.7 2.02 0.52 0.80 0.06 Example 3 17 0 50 0 21 0 10 0 2 6.5 5.29 0.56 0.83 0.05 Example 4 30 0 30 0 28 0 10 0 2 7.7 1.80 0.95 1.29 0.07 Example 5 10 0 55 0 23 0 10 0 2 7.0 9.90 0.60 0.87 0.05 Example 6 34 0 26 0 28 0 10 0 2 9.8 1.38 0.99 1.34 0.07 Example 7 6 0 60 0 22 0 10 0 2 6.5 18.00 0.55 0.80 0.05 Example 8 30 0 43 0 15 0 10 0 2 8.8 2.58 0.40 0.67 0.05 Example 9 30 0 23 0 35 0 10 0 2 8.7 1.38 1.40 1.80 0.08 Example 10 20 0 58 0 10 0 10 0 2 8.3 5.22 0.23 0.46 0.05 Example 11 29 0 23 0 46 0 0 0 2 11.1 1.43 1.87 1.87 0.08 Example 12 20 0 20 0 35 0 0 0 25 8.0 1.80 1.79 1.79 1.28 Example 13 0 25 45 0 18 0 10 0 2 7.9 2.77 0.47 0.73 0.05 Example 14 25 0 0 45 18 0 10 0 2 8.5 2.88 0.53 0.82 0.06 Example 15 20 0 53 0 0 15 10 0 2 13.0 4.77 0.37 0.62 0.05 Example 16 26 0 42 0 0 20 0 10 2 9.0 2.91 0.56 0.56 0.06 Example 17 33 0 37 0 18 0 10 0 2 1.2 2.02 0.52 0.80 0.06 Example 18 33 0 37 0 18 0 10 0 2 19.9 2.02 0.52 0.80 0.06 Example 19 15 0 57 0 6 0 20 0 2 5.0 6.84 0.15 0.63 0.05 Comparative 0 0 68 0 20 0 10 0 2 10.0 — 0.47 0.70 0.05 Example 1 Comparative 68 0 0 0 20 0 10 0 2 9.0 — 0.84 1.26 0.08 Example 2 Comparative 3 0 65 0 20 0 10 0 2 8.0 39.00 0.48 0.71 0.05 Example 3 Comparative 65 0 3 0 20 0 10 0 2 8.5 0.08 0.81 1.22 0.08 Example 4 Comparative 33 0 37 0 0 0 28 0 2 11.0 2.02 0.00 0.80 0.06 Example 5 Comparative 35 0 48 0 5 0 10 0 2 7.7 2.47 0.12 0.35 0.05 Example 6 Comparative 8 0 36 0 54 0 0 0 2 5.0 8.10 2.12 2.12 0.08 Example 7 Comparative Phosphate/soap treatment Example 8

(1-2) Pretreatment, Lubricating Coating Treatment, and Drying Treatment

A pretreatment, a lubricating coating treatment, and a drying treatment were carried out with respect to a surface of a φ12.5 mm steel wire rod (steel type: SCM435) by the following steps.

<Pretreatment, Lubricating Coating Treatment, and Drying Treatment for Producing Steel Wire Rods or Cold Rolled Steel Sheets of Examples 1 to 19 and Comparative Examples 1 to 7>

(a) Degreasing: commercially available degreasing agent (FINECLEANER E6400, manufactured by Nihon Parkerizing Co., Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10 minutes (b) Water rinsing: tap water, normal temperature, immersion: 30 seconds (c) Pickling: 17.5% hydrochloric acid, normal temperature, immersion: 20 minutes (d) Water rinsing: tap water, normal temperature, immersion: 30 seconds (e) Neutralization: commercially available neutralizer (PREPALENE 27, manufactured by Nihon Parkerizing Co., Ltd.) (f) Lubricating coating treatment: Each lubricating coating agent prepared in (1-1), temperature: 60° C., immersion: 1 minute (g) Drying: 100° C., 10 minutes (h) Mass of the coating is calculated from difference in mass between steel wire rods before and after treatment, and surface area.

<Pretreatment, Lubricating Coating Treatment, and Drying Treatment for Producing Steel Wire Rods or Cold Rolled Steel Sheets of Comparative Example 8>

(a) Degreasing: commercially available degreasing agent (FINECLEANER E6400, manufactured by Nihon Parkerizing Co., Ltd.), concentration: 20 g/L, temperature: 60° C., immersion: 10 minutes (b) Water rinsing: tap water, normal temperature, immersion: 30 seconds (c) Pickling: hydrochloric acid, concentration: 17.5%, normal temperature, immersion: 20 minutes (d) Water rinsing: tap water, normal temperature, immersion: 30 seconds (e) Chemical conversion coating: commercially available zinc phosphate chemical conversion treatment agent (PALBOND 3696X, manufactured by Nihon Parkerizing Co., Ltd.), concentration: 75 g/L, temperature 80° C., immersion: 10 minutes (f) Water rinsing: tap water, normal temperature, immersion: 30 seconds (g) Soap treatment: commercially available reactive soap lubricant (PALUBE 235, manufactured by Nihon Parkerizing Co., Ltd.), concentration: 70 g/L, temperature 85° C., immersion: 3 minutes (h) Drying: 100° C., 10 minutes (i) Amount of coating: 10 g/m²

(1-3) Evaluation Test (1-3-1) Heading Test

Using steel wire rod in size of φ12.5 mm×4 m subjected to the respective treatments in (1-2), heading was performed by the procedure shown in FIG. 1. First, the steel wire rod was drawn through a φ11.05 die to fabricate a φ11.05 steel wire rod. Thereafter, two-stage heading was carried out by a heading former. More specifically, as shown in the left figure of FIG. 1, the steel wire rod was cut to the proper length. The steel wire rod thus cut was formed into the dimension shown in the center figure of FIG. 1 at a first stage, and then formed into the size of φ7.2 mm at a second stage as shown in the right figure of FIG. 1. In the heading test, former oil (DiaPress No. 17B manufactured by Kansaiyushi Kogyo Co., Ltd.) was applied immediately before heading in all levels.

The coating retention amount of the steel wire rod after heading, and the presence of poor appearance were evaluated in the following manner.

(Coating Retention Amount after Heading)

The coating retention amount after heading was determined as follows: a coating was removed (film removal) by the following method, and then the coating retention amount was calculated from the weight of the steel wire rod before and after removal, after heading.

Coating film remover: commercially available alkali remover (FC-E6463, manufactured by Nihon Parkerizing Co., Ltd.), 20 g/L

Film removal method: The coating film remover was heated to a liquid temperature of 60° C. and a steel wire rod after heading was immersed in the remover for 60 minutes, and then a coating film was removed by rubbing with a sponge. Thereafter, rinsing with deionized water was performed and water was completely brown away by compressed air.

Evaluation criteria: The coating film retention amount was calculated in the following manner and then coating film retention properties were evaluated by the following criteria. The more the coating film retention amount increases, the more seizure resistance after heading becomes satisfactory. In this Example, the case where the coating retention amount is 0.8 g/m² or more was determined to be pass.

Coating retention amount (g/m²)=(weight of test piece before film removal−weight of test piece after film removal)/surface area of test piece

B: Coating retention amount is 1.8 g/m² or more. C: Coating retention amount is 0.8 g/m² or more and less than 1.8 g/m² D: Coating retention amount is less than 0.8 g/m² (Evaluation Criteria of Poor Appearance after Heading) B: No poor appearance occurs. D: Poor appearance occurs over the entire surface of the heading portion.

(1-3-2) Corrosion Resistance (Long-Term Rust Prevention Property) Test

In the steel wire rod after heading, it is difficult to evaluate a corrosion resistance test, so that a corrosion resistance test was performed using a steel rod on which a lubricating coating was formed of each lubricating coating agent. Specifically, SPCC-SD (75 mm×35 mm×0.8 mm) manufactured by Paltec Test Panels Co., Ltd. was subjected to each treatment in (1-2). In summer season, the steel wire rod was exposed to an open-air atmosphere indoors for two months to generate rust, and then the degree of rusting was observed. It was judged that the more the rust area increases, the more corrosion resistance (long-term rust prevention property) becomes inferior.

(Evaluation Criteria)

A: Extremely excellent as compared with performance of a lubricating coating in SPCC-SD of Comparative Example 8 (rust area of 3% or less) B: Excellent as compared with performance of a lubricating coating in SPCC-SD of Comparative Example 8 (rust area of exceeding 3% or 10% or less) C: Equivalent to performance of a lubricating coating in SPCC-SD of Comparative Example 8 (rust area of exceeding 10% or 20% or less)

D: Inferior as compared with performance of a lubricating coating in SPCC-SD of Comparative Example 8 (rust area of exceeding 20% or 30% or less)

These test results are shown in Table 2.

TABLE 2 Evaluation after heading Corrosion Coating retention properties resistance Coating retention Rust amount area Phosphorizing Appearance (g/m²) Evaluation (%) Evaluation property *1 Example 1 B 2.2 B 0 A B Example 2 B 2.2 B 0 A B Example 3 B 2.1 B 1 A B Example 4 B 2.0 B 0 A B Example 5 B 2.0 B 2 A B Example 6 B 1.7 C 4 B B Example 7 B 1.5 C 6 B B Example 8 B 2.1 B 0 A B Example 9 B 1.9 B 2 A B Example 10 B 2.1 B 0 A B Example 11 B 1.4 C 6 B B Example 12 B 1.5 C 7 B B Example 13 B 2.0 B 1 A B Example 14 B 2.1 B 1 A B Example 15 B 2.1 B 1 A B Example 16 B 2.0 B 1 A B Example 17 B 0.9 C 8 B B Example 18 B 1.7 C 0 A B Example 19 B 1.8 B 2 A B Comparative B 0.6 D 29 D B Example 1 Comparative B 0.7 D 26 D B Example 2 Comparative B 0.9 C 23 D B Example 3 Comparative B 1.4 C 22 D B Example 4 Comparative D 2.1 B 1 A B Examples Comparative D 2.1 B 0 A B Example 6 Comparative B 0.5 D 14 C B Example 7 Comparative B 1.7 C 12 C D Example 8 *1 B: There is no possibility of brittle fracture of steel wire rod due to phosphorus because of containing no phosphorus. D: There is possibility of brittle fracture of a steel wire rod due to phosphorus because of containing phosphorus.

First, all the steel wire rods of Examples 1 to 19 in Table 2 are steel wire rod or SPCC-SD having a lubricating coating which satisfy constituent features of the present disclosure and exhibited satisfactory appearance after heading, and had high corrosion resistance. Because of large coating retention amount after heading, no seizure occurs, and service life of the die is excellent.

To the contrary, the steel wire rods of Comparative Examples, which do not satisfy constituent features of the present disclosure, have the following problems.

First, the steel wire rod of Comparative Example 1 is a steel wire rod or SPCC-SD having a lubricating coating, which does not contain a water-soluble silicate as a source of supply of silicon (A). The steel wire rod of Comparative Example 2 is a steel wire rod or SPCC-SD having a lubricating coating, which does not contain a water-soluble tungstate as a source of supply of tungsten (B). The steel wire rod of Comparative Example 3 is a steel wire rod or SPCC-SD having a lubricating coating, in which a dry mass ratio (B)/(A) of A and B is high. The steel wire rod of Comparative Example 4 is a steel wire rod or SPCC-SD having a lubricating coating, in which a dry mass ratio (B)/(A) of A and B is low. All the steel wire rods of Comparative Examples 1 to 4 were inferior in corrosion resistance.

The steel wire rods of Comparative Examples 5 to 7 are steel wire rods having a lubricating coating or SPCC-SD, in which the dry mass ratio of (C)/{(A)+(B)} does not satisfy the scope of the present disclosure, or steel wire rods or SPCC-SD having a lubricating coating, in which the dry mass ratio does not satisfy the scope of the present disclosure, and an alkali metal salt of fatty acid (C) is not contained.

Among these, the steel wire rod of Comparative Example 5 is a steel wire rod or SPCC-SD having a lubricating coating, which does not contain an alkali metal salt of fatty acid (C) and contains only a wax (D) as the lubricant, was inferior in heading appearance. These test results reveal that, to obtain excellent appearance after heading, it is indispensable that an alkali metal salt of fatty acid (C) is contained in the lubricating coating, thus failing to obtain desired effect even when wax as a typical lubricant is added.

The steel wire rod of Comparative Example 6 is a steel wire rod or SPCC-SD having a lubricating coating, in which the dry mass ratio of (C)/{(A)+(B)} is lower than the scope of the present disclosure. Since the addition effect of an alkali metal salt of fatty acid (C) is not effectively exerted, heading appearance was poor.

The steel wire rod of Comparative Example 7 is a steel wire rod or SPCC-SD having a lubricating coating, in which the dry mass ratio of (C)/{(A)+(B)} is higher than the scope of the present disclosure. Due to the addition effect of an alkali metal salt of fatty acid (C), appearance after heading was satisfactory. Because of too large amount of an alkali metal salt of fatty acid, the coating retention amount after heading decreased, leading to the occurrence of microseizure.

The steel wire rod of Comparative Example 8 is a steel wire rod or SPCC-SD (Conventional Example) having a phosphate coating formed by performing a reaction soap treatment. When heat treatments such as hardening and tempering are performed, phosphorizing may occur, leading to brittle steel wire rod.

As is apparent from the above description, the steel wire rod of the present disclosure includes no phosphorus in a lubricating coating, and is therefore free from phosphorizing property, and has satisfactory long-term corrosion resistance, which is identical to or higher than that of a conventional phosphate and a soap-treated material, and is also free from poor appearance after heading. Therefore, the steel wire rod of the present disclosure has significantly high industrial value of utilization.

The content of disclosure of the description include the following aspects.

Aspect 1:

A steel wire rod including a lubricating coating on a surface thereof, wherein the lubricating coating including silicon (A), tungsten (B) and an alkali metal salt of fatty acid (C), a dry mass ratio of (B)/(A) is in a range of 1.3 to 18 and a dry mass ratio of (C)/{(A)+(B)} is in a range of 0.14 to 2.0, and the lubricating coating contains no phosphorus.

Aspect 2:

The steel wire rod according to aspect 1, wherein the silicon is derived from a water-soluble silicate, and the tungsten is derived from a water-soluble tungstate.

Aspect 3:

The steel wire rod according to aspect 1 or 2, wherein the silicon is derived from at least one selected from the group consisting of lithium silicate, sodium silicate, and potassium silicate, and the tungsten is derived from at least one selected from the group consisting of lithium tungstate, sodium tungstate, potassium tungstate, and ammonium tungstate.

Aspect 4:

The steel wire rod according to any one of aspects 1 to 3, wherein the lubricating coating further includes a lubricant (D) other than the alkali metal salt of fatty acid (C), and a dry mass ratio of {(C)+(D)}/{(A)+(B)} is in a range of 0.14 to 2.0.

Aspect 5:

The steel wire rod according to aspect 4, wherein the lubricant (D) is at least one selected from the group consisting of wax, polytetrafluoroethylene, fatty acid metal soap, fatty acid amide, molybdenum disulfide, tungsten disulfide, graphite, and melamine cyanurate.

Aspect 6:

The steel wire rod according to any one of aspects 1 to 5, wherein the lubricating coating further includes a resin (E), and a dry mass ratio of (E)/{(A)±(B)} exceeds 0 and 1.4 or less.

Aspect 7:

The steel wire rod according to aspect 6, wherein the resin (E) is at least one selected from the group consisting of a vinyl resin, an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a cellulose derivative, a polymaleic acid, and a polyester resin.

Aspect 8:

The steel wire rod according to any one of aspects 1 to 7, wherein a mass of a coating per unit area of the lubricating coating is in a range of 1.0 to 20 g/m².

This application claims priority based on Japanese Patent Application No. 2015-195149 filed on Sep. 30, 2015, and Patent Application No. 2016-121490 filed on Jun. 20, 2016, the disclosure of which is incorporated by reference herein. 

1: A steel wire rod comprising a lubricating coating on a surface thereof, wherein the lubricating coating comprises: (A) silicon (A); (B) tungsten (B); and (C) an alkali metal salt of fatty acid (C), wherein: a dry mass ratio of (B)/(A) ranges from 1.3 to 18; a dry mass ratio of (C)/{(A)+(B)} ranges from 0.14 to 2.0; and the lubricating coating contains no phosphorus. 2: The steel wire rod according to claim 1, wherein the silicon (A) is derived from a water-soluble silicate; and the tungsten (B) is derived from a water-soluble tungstate. 3: The steel wire rod according to claim 1, wherein: the silicon (A) is derived from at least one selected from the group consisting of lithium silicate, sodium silicate and potassium silicate; and the tungsten (B) is derived from at least one selected from the group consisting of lithium tungstate, sodium tungstate, potassium tungstate and ammonium tungstate. 4: The steel wire rod according to claim 1, wherein; the lubricating coating further comprises: (D) a lubricant (D) other than the alkali metal salt of fatty acid (C); and a dry mass ratio of {(C)+(D)}/{(A)+(B)} ranges from 0.14 to 2.0. 5: The steel wire rod according to claim 4, wherein the lubricant (D) is at least one selected from the group consisting of a wax, a polytetra fluoroethylene, a fatty acid metal soap, a fatty acid amide, molybdenum disulfide, tungsten disulfide, graphite, and melamine cyanurate. 6: The steel wire rod according to claim 1, wherein the lubricating coating further comprises: (E) a resin (E); and a dry mass ratio of (E)/{(A)+(B)} exceeds 0 and is 1.4 or less. 7: The steel wire rod according to claim 6, wherein the resin (E) is at least one selected from the group consisting of a vinyl resin, an acrylic resin, an epoxy resin, a urethane resin, a phenol resin, a cellulose derivative, a polymaleic acid, and a polyester resin. 8: The steel wire rod according to claim 1, wherein a mass of a coating per unit area of the lubricating coating ranges from 1.0 to 20 g/m². 